The present invention relates to the communication of digital data, and more particularly to a system for communicating program data over a cable television network or the like together with program specific tag data and nonprogram specific receiver control data. The invention has particular applicability to the communication of high quality digital audio signals via a cable television system.
Cable television ("CATV") networks can carry various types of information. Television programs comprise the bulk of the information currently carried via CATV systems. Such information has generally been carried in an analog form, modulated on a radio frequency ("RF") signal for recovery and reproduction by a subscriber's television receiver.
Cable television growth has come from the development of various programming categories and by the technologies that have made program delivery possible. Cable first brought distant television signals to areas where there was little or no off-air reception. This applies to distant signals and weak signal areas where outdoor antennas were previously mandatory. The next category to bring major growth to cable was pay service after the development of reasonable cost satellite delivery systems. Such services require a subscriber to pay a monthly service fee to the cable system operator in order to receive a desired level of basic and optional premium programming. Recently, addressable technology and aggressive marketing have enabled "pay-per-view" programming to proliferate.
The transmission of digital data via a cable television network is also possible. Recent developments in this area include the provision of high quality digital audio services over cable. In the past, the signal quality of analog audio transmission techniques has been poor and there has been no efficient way to collect revenue or control access to such services. With digital techniques, access to premium audio programming can be controlled so that only authorized subscribers can recover and reproduce the audio data. Digital audio broadcasts provide reproduction of music that is extremely realistic and absent from background noise and distortions that have plagued other high fidelity sound systems in wide scale use.
An example of high fidelity sound reproduction using digital techniques can be found in the compact disc technology that has enjoyed tremendous success as an alternative to phonograph records and tapes. Digital techniques for the communication of high fidelity audio programs via a CATV network as well as via direct broadcast satellite and over the air enable the provision of high quality audio services on a subscription basis. Commonly owned U.S. patent application Ser. No. 07/280,770 filed Dec. 6, 1988 for "Apparatus and Method for Providing Digital Audio in the FM Broadcast Band", incorporated herein by reference, discloses a method and apparatus for providing high quality digital sound signals within the FM broadcast band. Commonly owned U.S. Pat. No. 4,821,097 to C. Robbins entitled "Apparatus and Method for Providing Digital Audio on the Sound Carrier of a Standard Television Signal", also incorporated herein by reference, discloses a system wherein the FM audio portion of a standard television signal is replaced with digital audio.
In the techniques disclosed in the above-mentioned references, the digital audio information is carried using multilevel (e.g., multiphase) modulation. A well known type of multiphase modulation is quadrature phase shift keyed ("QPSK") modulation. Advantages of QPSK modulation are discussed in C. Robbins, "Digital Audio for Cable Television", 1986 NCTA Technical Papers, Dallas, Tex., Mar. 15-18, 1986, pp. 21-24.
In a QPSK transmission, an in-phase component I and an out-of-phase component Q (typically, 90.degree. out of phase with the I component) are provided. The I and Q components are processed in a well known manner to recover the original digital data that was used to modulate the QPSK signal. In the past, the I and Q components have been detected from the transmitted signal on which they are carried using a phase reference derived independently of the modulated data carrier. An example of such a demodulator is provided in FIG. 6 of the above-mentioned U.S. Pat. No. 4,821,097.
In order to control access to program data carried on a cable television or satellite television system, the data is encrypted as described, for example, in U.S. Pat. No. 4,613,901 entitled "Signal Encryption and Distribution System for Controlling Scrambling and Selective Remote Descrambling of Television Signals" to Gilhousen, et al, and U.S. Pat. No. 4,563,702 entitled "Video Signal Scrambling and Descrambling Systems" to Heller, et al. These systems provide for digital stereo audio transmission in the horizontal blanking interval of a National Television Standards Committee ("NTSC") television signal. Other, less sophisticated scrambling systems for the video portions of television signals are well known.
The encryption of digital data for transmission via a subscription television network requires the insertion of various cryptographic key information and control data within a data stream carrying the program data. Some of the data, commonly referred to as "tag data" is specific to a particular program being communicated. Other data, sometimes referred to as "addressable control data" or "receiver control data" is not specific to any particular program, but may be specific to a particular addressable subscriber terminal, such as a set top converter coupled between the incoming cable and the subscriber's television set.
In the case of digital audio data, which is advantageously encoded using the Dolby.RTM. adaptive delta modulation ("ADM") encoding system, the digital audio data must also be accompanied by step data and emphasis data necessary to reproduce the original audio program. The integration of these various categories of data (i.e., tag data, receiver control data, encryption data, step data and emphasis data) for transmission is a complicated engineering problem that can impact the ultimate cost of the communication system components.
It would be advantageous to provide a system for communicating different categories of digital data in a single data stream. Since any communication system will consist of only a few transmitters but literally millions of receivers, it would be further advantageous to configure the data stream format such that the data can be recovered using receivers that are relatively inexpensive to manufacture and install. The present invention provides a system and receiver apparatus enjoying the aforementioned advantages.